The present invention concerns a a learning sequence for estimating a transmission channel. It also concerns a sequence of symbols comprising data symbols and a a learning sequence of this kind and a device for estimating a transmission channel adapted to operate with a learning sequence of this kind.
In a transmission system, especially a radio transmission system, a transmitter transmits a sequence of symbols to a receiver in a transmission channel. The sequence transmitted is degraded by the transmission channel with the result that the sequence of symbols received by the receiver is no longer identical to the transmitted sequence. The main deterioration is intersymbol interference caused by the fact that a transmitted symbol can take more than one path in the transmission channel. If the distance between two paths at least is greater than the distance between two symbols transmitted in succession, a symbol taking one of these paths can interfere with a subsequent symbol taking another, shorter path.
An equalizer is used in the receiver to correct intersymbol interference. To operate correctly it must know the impulse response of the transmission channel. To this end, special symbols are transmitted in a learning sequence. These symbols are known to the receiver, unlike the data symbols which are transmitted and which it may be assumed are unknown to the receiver. It is therefore routine practice for a packet of symbols addressed to a particular receiver to comprise in succession, data symbols, a learning sequence and further data symbols, so that the learning sequence is at the centre of the packet.
The learning sequence is chosen to suit the specifications of the transmission channel and in particular its length.
Given that the symbols are transmitted regularly and therefore have a period, known as the symbol period the length of the channel is defined as the number of symbol periods equivalent to the difference between the longest path and the shortest path of the channel.
To establish the impulse response of the channel, a channel estimator device is used in the receiver to generate a replica of the learning sequence and to correlate it with the respective sequence of symbols received. The result of the correlation is a set of coefficients h.sub.i where i varies from 0 to L, L being the length of the channel, the set of coefficients being addressed to the equalizer. The most direct path of the channel is represented by h.sub.0, and the other coefficients represent longer paths causing interference with the the most direct path.
In the most general case these coefficients are complex because the symbols received have two orthogonal components, a "phase" component and a "quadrature" component. By convention this set of coefficients is called the impulse response.
The learning sequence always comprises a first guard sequence followed by a reference sequence followed by a second guard sequence. The first guard sequence is formed with the last L symbols of the reference sequence while the second guard sequence is formed with the first L symbols of the reference sequence. Because of the uncertainty as to the exact position of the learning sequence in the succession of symbols received, the correlation which applies only to the reference sequence is effected on a received symbol sequence of the same length but whose origin is displaced until the reference sequence generated locally is coincident with the corresponding received symbol sequence; the guard sequences are thus disposed to prevent correlation between the local reference sequence and received data symbols. For further information on the learning sequences used, which all have the same structure, reference may be had to the following documents, for example:
Giovanna d'ARIA, Valerio ZINGARELLI: "Design and performance of synchronization techniques and Viterbi adaptative equalizers for narrowband TDMA mobile radio", Proceedings DMR III, Copenhagen, 13-15 September 1988;
Dirk POPPEN: "Design of training sequence for channel impulse response measurement", COST 231, Vienna, Austria, pp 2-10, January 1992;
GSM recommendations (series 05) defining the paneuropean digital cellular mobile radio network.
It seems that to use such learning sequences it is necessary to reserve 2L symbols to form the two guard sequences. Thus these symbols cannot be used to transmit data, which reduces the quantity of data that can be transmitted in a packet.